Orientation-dependent imaging of electronically excited quantum dots

We previously demonstrated that we can image electronic excitations of quantum dots by single-molecule absorption scanning tunneling microscopy (SMA-STM). With this technique, a modulated laser beam periodically saturates an electronic transition of a single nanoparticle, and the resulting tunneling current modulation ΔI(x0, y0) maps out the SMA-STM image. In this paper, we first derive the basic theory to calculate ΔI(x0, y0) in the one-electron approximation. For near-resonant tunneling through an empty orbital “i” of the nanostructure, the SMA-STM signal is approximately proportional to the electron density φix0,y02 of the excited orbital in the tunneling region. Thus, the SMA-STM signal is approximated by an orbital density map (ODM) of the resonantly excited orbital at energy Ei. The situation is more complex for correlated electron motion, but either way a slice through the excited electronic state structure in the tunneling region is imaged. We then show experimentally that we can nudge quantum dot...